Characteristic features of fiber lasers include high output beam quality, compact size, ease-of-use, and low running cost. Fiber lasers can generate either continuous-wave (CW) radiation or pulse radiation. Pulsed operation can be achieved via Q-switching techniques. Q-switched fiber lasers are preferred for applications such as micro-machining, marking, and scientific research due to their high peak power and excellent beam quality. Q-switching is achieved by inserting a fast optical switch in the laser resonance cavity to control optical loss in the cavity. In particular, the optical switch must have fast speed, low insertion loss and high switch extinction ratio. Initially, the laser cavity is kept on a low Q factor state, laser oscillation cannot occur at this initial period, but energy from a pump source accumulates in the gain medium. Subsequently, the laser cavity is switched to a high Q factor state, so that laser oscillation builds up quickly in the cavity and generates a high peak power laser pulse. When the laser cavity is switched between low Q and high Q by the optical switch, sequenced laser pulses are produced.
Optical switching for Q-switching can be achieved by either active or passive means. Examples of active Q-switching means include acousto-optic modulators (AOMs) and electro-optic modulators (EOMs). The AOM comprises optical crystals such as tellurium dioxide, crystalline quartz, and fused silica. The EOM comprises optical materials such as potassium di-deuterium phosphate (KD*P), beta barium borate (BBO), lithium niobate (LiNbO3), as well as NH4H2PO4 (ADP), and other materials. One drawback of known AOM and EOM devices is that they are relatively bulky. This is a drawback particularly for Q-switching in fiber laser system, because the fibre core has a relatively small diameter, the difference of which relative to the size of the modulator complicates light coupling between the device and an optical fiber. Further, AOM and EOM devices are relatively expensive.
A typical configuration of a Q-switched fiber laser is illustrated in FIG. 1. The laser cavity comprises a pair of fiber Bragg grating (FBG) reflectors (15, 35) having the same center wavelength, a gain fiber (18) which provides optical gain, and an optical switch (90) coupled to an optical fiber pigtail (20) for coupling a light signal between the fiber and the switch. The optical switch may be either an AOM or EOM type. A pump source (1) provides pump light (5) which is coupled to the fiber laser cavity to excite the gain fiber (18). The FBG reflectors provide optical feedback for laser oscillation. The optical switch (90) is employed as a switch to control optical loss within the laser cavity, and thereby provide Q-switching. Initially, the cavity loss is kept on a high level with the switch “off” (low Q factor state of the laser cavity), at which time no light signal passes through the switch (90). As discussed above, laser oscillation does not occur at this time, but energy from pump light source (5) accumulates in the gain fiber (18). Subsequently, the cavity loss is reduced over a relatively short time by “switching on” the optical switch to a low loss level (high Q factor state of the laser cavity), at which time the light signal passes through optical switch (90). Consequently, laser oscillation builds up quickly in the cavity and generates a high peak power laser pulse. The FBG pair (15, 35) have the same center wavelength and function as narrow band reflective mirrors which provide optical feedback to the laser cavity and confine the laser oscillation wavelength to the FBG wavelength. Since the FBG has a relatively narrow reflective bandwidth, the laser oscillates only at this wavelength and the output has a narrow wavelength spectrum. When the laser cavity is switched sequentially between the low Q factor state and the high Q factor state by means of the optical switch (90), sequenced laser pulses are produced. Switch control is achieved by means of a signal (95) from an external controller (96). One device of the FBG pair (15, 35) is partially transparent and has relatively lower reflectivity, resulting in a percentage of the generated laser light being permitted to leave laser cavity and deliver the laser output (38 or 42).
Referring to FIG. 2a, the FBG is formed by introducing a periodic changes of refractive index in the fiber core. The modified area (151) within the fiber core has a smaller refractive index difference of period ΛB relative to the adjacent unmodified area (152). Several techniques are known for changing the refractive index of discreet areas of the fibre core. One technique is to expose the area to a UV laser beam, e.g., area (151) is altered by exposure to UV light, but area (152) is neither exposed nor altered.
The principle characteristic parameters of a FBG are center wavelength λB, bandwidth ΔλB, and reflectivity. The condition for high reflection, known as the Bragg condition, relates the reflected wavelength, or Bragg wavelength, λB to the grating period ΛB and the effective refractive index of the fiber core n via:λB=2nΛB.FIGS. 2b, 2c, and 2d illustrate the spectral characteristics of a FBG. When broad band light (110, FIG. 2a) having spectrum (120, FIG. 2b) is input into the FBG as shown, the reflected light (112, FIG. 2a) has a corresponding spectrum (122, FIG. 2c), and the transmitted light (111, FIG. 2a) has a corresponding spectrum (121, FIG. 2d).
Somewhat similar to the FBG in terms of physical configuration, a Long Period Fiber Grating (LPFG) has a grating period ΛL which is considerably longer than the period ΛB of the FBG, i.e., typically ΛL is 200˜2000 times longer than ΛB. The LPFG couples the fundamental mode in the fiber core with the cladding modes of the fiber and propagates them in the same direction. The excited cladding modes are attenuated, resulting in the appearance of resonance loss in the transmission spectrum. However, in contrast with the FBG, the LPFG does not produce reflected light. FIGS. 3a, 3b and 3c illustrate the physical configuration and the spectral transmission characteristics of a LPFG. The periodic grating structure (22, FIG. 3a) can be made by using a UV laser beam to “burn” discreet, periodically spaced areas in the fiber core in a manner which is similar to that described above with reference to the FBG, where the modified area (251) exhibits a refractive index change in comparison with unmodified area (252). Recent research suggests that the modified areas can be also formed by using a high voltage electric arc discharge or CO2 laser to “burn” the fiber, i.e., introducing structural changes and slight geometrical deformation in the irradiated area of the fibre. Mechanical stress can be used, e.g., by applying static stress to the areas of the fibre to be modified through a corrugated plate. The refractive index at the areas subjected to stress is changed in accordance with the photo-elastic effect, but the adjacent areas which are not subjected to stress are unmodified.
When a broad band light (210, FIG. 3a) having spectrum (220, FIG. 3b) is input into the LPFG, the transmitted light (211, FIG. 3a) has a corresponding spectral characteristic (221, FIG. 3c), several resonance loss peaks (222, 223), including the fundamental mode coupling with different cladding modes of the fiber. However, there is no light reflection. Considering resonance loss peak (222, FIG. 3c), having a center wavelength XL, and bandwidth ΔλL, the resonance loss of the LPFG is due to the coupling of the fundamental mode in the fiber core with the cladding modes of the fiber. The phase matching between the fundamental mode and cladding modes at wavelength λmL can be expressed as:λmL=(ncore−nclm)ΛL,where ncore is the effective refractive index of the fundamental mode, nclm is the effective refractive index of the mth cladding mode, and ΛL is the period of the LPFG. Since several cladding modes can satisfy this condition, each one is at a different center wavelength λmL, and thus the transmission spectrum of the LPFG exhibits a series of transmission loss notch peaks (222, 223, FIG. 3C).
FIGS. 4a-4c illustrate the physical configuration and the spectral transmission characteristics of a phase shifted LPFG. In the phase shifted LPFG, a part of the grating period is shifted at the grating center by Λp. As a result, a phase shift is introduced into the LPFG. For example, by introducing a π-phase shift at the center of the LPFG, the notch peak (See FIG. 3c) is changed to a reverse peak (232, FIG. 4c). For a broad band input (220, FIG. 4b), a corresponding transmission spectrum (231, FIG. 4c) of the phase shifted LPFG is produced, enabling transmission at wavelength XL.
FIGS. 5a-5c illustrate the physical configuration and the spectral transmission characteristics of cascaded LPFGs. Cascaded LPFGs are formed by connecting a pair of LPFGs (25, 26) in series. Each of the LPFGs has a grating length d1 and d2, and together define a separation distance of L. When broad band light (210) having spectrum (220, FIG. 5b) is input into the cascaded LPFGs, the corresponding transmitted light (211) has a corresponding spectral transmission response (241, FIG. 5c). It can be seen from FIGS. 5b and 5c that the spectrum of the transmitted light has several spectral transparent peaks (242, 244 and 246) and several spectral loss peaks (245, 243). This is due to interference between the fundamental mode and cladding modes. The first LPFG couples part of the fundamental mode to the cladding modes, and then the coupled cladding modes and fundamental mode travel along the fiber simultaneously to the second LPFG. At the second LPFG, the two modes interact with each other and generate spectral interference fringe patterns. The fringe spacing ΔλPL is related to the grating length d1, d2, d and the separation distance L between the two LPFGs. An increase in L corresponds with a decrease in the fringe spacing ΔλPL. For multi-channel filter applications the distance L is typically less than 600 mm.